Mutations of the adenomatous polyposis coli (APC) gene are the most common disease-causing genetic events in humans; approximately 50% of the population will develop colorectal polyps initiated by such mutations during a normal life span (14). Individuals who inherit APC mutations develop thousands of colorectal tumors, consistent with APC's tumor suppressor or "gatekeeping" role in colorectal tumorigenesis (15,16). APC homodimerizes through its amino-terminus (17), and interacts with at least six other proteins: .beta.-catenin (18), .gamma.-catenin (plakoglobin) (19), tubulin (20), EB1 (21), hDLG, a homologue of a Drosophila tumor suppressor protein (22), and ZW3/GSK3.beta. kinase (23). Whether any of these interacting proteins communicate APC growth-controlling signals is unknown. Thus there is a need in the art for a fuller understanding of how the tumor suppressor gene APC functions in cells.